International Institute for Applied Systems Analysis Schlossplatz 1
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Interim Report IR-06-075
The role of trade-off shapes in the evolution of parasites in spatial host populations: an approximate analytical approach
Masashi Kamo (masashi-kamo@aist.go.jp)
Akira Sasaki (asasascb@mbox.nc.kyushu-u.ac.jp) Mike Boots (m.boots@sheffield.ac.uk)
Approved by Ulf Dieckmann
Program Leader, Evolution and Ecology Program December 2006
IIASA S TUDIES IN A DAPTIVE D YNAMICS N O. 128
EEP
The Evolution and Ecology Program at IIASA fosters the devel- opment of new mathematical and conceptual techniques for un- derstanding the evolution of complex adaptive systems.
Focusing on these long-term implications of adaptive processes in systems of limited growth, the Evolution and Ecology Program brings together scientists and institutions from around the world with IIASA acting as the central node.
Scientific progress within the network is collected in the IIASA Studies in Adaptive Dynamics series.
No. 1 Metz JAJ, Geritz SAH, Meszéna G, Jacobs FJA, van Heerwaarden JS: Adaptive Dynamics: A Geometrical Study of the Consequences of Nearly Faithful Reproduction. IIASA Working Paper WP-95-099 (1995). van Strien SJ, Verduyn Lunel SM (eds): Stochastic and Spatial Structures of Dynami- cal Systems, Proceedings of the Royal Dutch Academy of Sci- ence (KNAW Verhandelingen), North Holland, Amsterdam, pp. 183-231 (1996).
No. 2 Dieckmann U, Law R: The Dynamical Theory of Co- evolution: A Derivation from Stochastic Ecological Processes.
IIASA Working Paper WP-96-001 (1996). Journal of Mathe- matical Biology 34:579-612 (1996).
No. 3 Dieckmann U, Marrow P, Law R: Evolutionary Cy- cling of Predator-Prey Interactions: Population Dynamics and the Red Queen. IIASA Preprint (1995). Journal of Theoreti- cal Biology 176:91-102 (1995).
No. 4 Marrow P, Dieckmann U, Law R: Evolutionary Dy- namics of Predator-Prey Systems: An Ecological Perspective.
IIASA Working Paper WP-96-002 (1996). Journal of Mathe- matical Biology 34:556-578 (1996).
No. 5 Law R, Marrow P, Dieckmann U: On Evolution under Asymmetric Competition. IIASA Working Paper WP-96-003 (1996). Evolutionary Ecology 11:485-501 (1997).
No. 6 Metz JAJ, Mylius SD, Diekmann O: When Does Evo- lution Optimize? On the Relation Between Types of Density Dependence and Evolutionarily Stable Life History Parame- ters. IIASA Working Paper WP-96-004 (1996).
No. 7 Ferrière R, Gatto M: Lyapunov Exponents and the Mathematics of Invasion in Oscillatory or Chaotic Popula- tions. Theoretical Population Biology 48:126-171 (1995).
No. 8 Ferrière R, Fox GA: Chaos and Evolution. IIASA Preprint (1996). Trends in Ecology and Evolution 10:480- 485 (1995).
No. 9 Ferrière R, Michod RE: The Evolution of Cooperation in Spatially Heterogeneous Populations. IIASA Working Pa- per WP-96-029 (1996). The American Naturalist 147:692- 717 (1996).
No. 11 Geritz SAH, Metz JAJ, Kisdi É, Meszéna G: The Dy- namics of Adaptation and Evolutionary Branching. IIASA Working Paper WP-96-077 (1996). Physical Review Letters 78:2024-2027 (1997).
No. 12 Geritz SAH, Kisdi É, Meszéna G, Metz JAJ: Evo- lutionary Singular Strategies and the Adaptive Growth and Branching of the Evolutionary Tree. IIASA Working Paper WP-96-114 (1996). Evolutionary Ecology 12:35-57 (1998).
No. 13 Heino M, Metz JAJ, Kaitala V: Evolution of Mixed Maturation Strategies in Semelparous Life-Histories: The Crucial Role of Dimensionality of Feedback Environment.
IIASA Working Paper WP-96-126 (1996). Philosophi- cal Transactions of the Royal Society of London Series B 352:1647-1655 (1997).
No. 14 Dieckmann U: Can Adaptive Dynamics Invade?
IIASA Working Paper WP-96-152 (1996). Trends in Ecol- ogy and Evolution 12:128-131 (1997).
No. 15 Meszéna G, Czibula I, Geritz SAH: Adaptive Dynam- ics in a 2-Patch Environment: A Simple Model for Allopatric and Parapatric Speciation. IIASA Interim Report IR-97-001 (1997). Journal of Biological Systems 5:265-284 (1997).
No. 16 Heino M, Metz JAJ, Kaitala V: The Enigma of Frequency-Dependent Selection. IIASA Interim Report IR- 97-061 (1997). Trends in Ecology and Evolution 13:367-370 (1998).
No. 17 Heino M: Management of Evolving Fish Stocks.
IIASA Interim Report IR-97-062 (1997). Canadian Journal of Fisheries and Aquatic Sciences 55:1971-1982 (1998).
No. 18 Heino M: Evolution of Mixed Reproductive Strategies in Simple Life-History Models. IIASA Interim Report IR-97- 063 (1997).
No. 19 Geritz SAH, van der Meijden E, Metz JAJ: Evolution- ary Dynamics of Seed Size and Seedling Competitive Ability.
IIASA Interim Report IR-97-071 (1997). Theoretical Popu- lation Biology 55:324-343 (1999).
No. 20 Galis F, Metz JAJ: Why Are There So Many Cichlid
No. 21 Boerlijst MC, Nowak MA, Sigmund K: Equal Pay for all Prisoners/ The Logic of Contrition. IIASA Interim Report IR-97-073 (1997). American Mathematical Society Monthly 104:303-307 (1997). Journal of Theoretical Biology 185:281-293 (1997).
No. 22 Law R, Dieckmann U: Symbiosis Without Mutualism and the Merger of Lineages in Evolution. IIASA Interim Re- port IR-97-074 (1997). Proceedings of the Royal Society of London Series B 265:1245-1253 (1998).
No. 23 Klinkhamer PGL, de Jong TJ, Metz JAJ: Sex and Size in Cosexual Plants. IIASA Interim Report IR-97-078 (1997).
Trends in Ecology and Evolution 12:260-265 (1997).
No. 24 Fontana W, Schuster P: Shaping Space: The Possi- ble and the Attainable in RNA Genotype-Phenotype Mapping.
IIASA Interim Report IR-98-004 (1998). Journal of Theoret- ical Biology 194:491-515 (1998).
No. 25 Kisdi É, Geritz SAH: Adaptive Dynamics in Allele Space: Evolution of Genetic Polymorphism by Small Muta- tions in a Heterogeneous Environment. IIASA Interim Report IR-98-038 (1998). Evolution 53:993-1008 (1999).
No. 26 Fontana W, Schuster P: Continuity in Evolution: On the Nature of Transitions. IIASA Interim Report IR-98-039 (1998). Science 280:1451-1455 (1998).
No. 27 Nowak MA, Sigmund K: Evolution of Indirect Reci- procity by Image Scoring/ The Dynamics of Indirect Reci- procity. IIASA Interim Report IR-98-040 (1998). Nature 393:573-577 (1998). Journal of Theoretical Biology 194:561- 574 (1998).
No. 28 Kisdi É: Evolutionary Branching Under Asymmetric Competition. IIASA Interim Report IR-98-045 (1998). Jour- nal of Theoretical Biology 197:149-162 (1999).
No. 29 Berger U: Best Response Adaptation for Role Games.
IIASA Interim Report IR-98-086 (1998).
No. 30 van Dooren TJM: The Evolutionary Ecology of Dominance-Recessivity. IIASA Interim Report IR-98-096 (1998). Journal of Theoretical Biology 198:519-532 (1999).
No. 31 Dieckmann U, O’Hara B, Weisser W: The Evolution- ary Ecology of Dispersal. IIASA Interim Report IR-98-108 (1998). Trends in Ecology and Evolution 14:88-90 (1999).
No. 32 Sigmund K: Complex Adaptive Systems and the Evo- lution of Reciprocation. IIASA Interim Report IR-98-100 (1998). Ecosystems 1:444-448 (1998).
No. 33 Posch M, Pichler A, Sigmund K: The Efficiency of Adapting Aspiration Levels. IIASA Interim Report IR-98- 103 (1998). Proceedings of the Royal Society London Series B 266:1427-1435 (1999).
No. 34 Mathias A, Kisdi É: Evolutionary Branching and Co- existence of Germination Strategies. IIASA Interim Report IR-99-014 (1999).
No. 35 Dieckmann U, Doebeli M: On the Origin of Species by Sympatric Speciation. IIASA Interim Report IR-99-013 (1999). Nature 400:354-357 (1999).
No. 36 Metz JAJ, Gyllenberg M: How Should We Define Fit- ness in Structured Metapopulation Models? Including an Ap-
No. 37 Gyllenberg M, Metz JAJ: On Fitness in Structured Metapopulations. IIASA Interim Report IR-99-037 (1999).
Journal of Mathematical Biology 43:545-560 (2001).
No. 38 Meszéna G, Metz JAJ: Species Diversity and Popula- tion Regulation: The Importance of Environmental Feedback Dimensionality. IIASA Interim Report IR-99-045 (1999).
No. 39 Kisdi É, Geritz SAH: Evolutionary Branching and Sympatric Speciation in Diploid Populations. IIASA Interim Report IR-99-048 (1999).
No. 40 Ylikarjula J, Heino M, Dieckmann U: Ecology and Adaptation of Stunted Growth in Fish. IIASA Interim Report IR-99-050 (1999). Evolutionary Ecology 13:433-453 (1999).
No. 41 Nowak MA, Sigmund K: Games on Grids. IIASA Interim Report IR-99-038 (1999). Dieckmann U, Law R, Metz JAJ (eds): The Geometry of Ecological Interactions:
Simplifying Spatial Complexity, Cambridge University Press, Cambridge, UK, pp. 135-150 (2000).
No. 42 Ferrière R, Michod RE: Wave Patterns in Spatial Games and the Evolution of Cooperation. IIASA Interim Report IR-99-041 (1999). Dieckmann U, Law R, Metz JAJ (eds): The Geometry of Ecological Interactions: Simplifying Spatial Complexity, Cambridge University Press, Cambridge, UK, pp. 318-332 (2000).
No. 43 Kisdi É, Jacobs FJA, Geritz SAH: Red Queen Evo- lution by Cycles of Evolutionary Branching and Extinction.
IIASA Interim Report IR-00-030 (2000). Selection 2:161- 176 (2001).
No. 44 Meszéna G, Kisdi É, Dieckmann U, Geritz SAH, Metz JAJ: Evolutionary Optimisation Models and Matrix Games in the Unified Perspective of Adaptive Dynamics. IIASA Interim Report IR-00-039 (2000). Selection 2:193-210 (2001).
No. 45 Parvinen K, Dieckmann U, Gyllenberg M, Metz JAJ:
Evolution of Dispersal in Metapopulations with Local Density Dependence and Demographic Stochasticity. IIASA Interim Report IR-00-035 (2000). Journal of Evolutionary Biology 16:143-153 (2003).
No. 46 Doebeli M, Dieckmann U: Evolutionary Branch- ing and Sympatric Speciation Caused by Different Types of Ecological Interactions. IIASA Interim Report IR-00-040 (2000). The American Naturalist 156:S77-S101 (2000).
No. 47 Heino M, Hanski I: Evolution of Migration Rate in a Spatially Realistic Metapopulation Model. IIASA Interim Report IR-00-044 (2000). The American Naturalist 157:495- 511 (2001).
No. 48 Gyllenberg M, Parvinen K, Dieckmann U: Evolution- ary Suicide and Evolution of Dispersal in Structured Metapop- ulations. IIASA Interim Report IR-00-056 (2000). Journal of Mathematical Biology 45:79-105 (2002).
No. 49 van Dooren TJM: The Evolutionary Dynamics of Di- rect Phenotypic Overdominance: Emergence Possible, Loss Probable. IIASA Interim Report IR-00-048 (2000). Evolu- tion 54:1899-1914 (2000).
No. 50 Nowak MA, Page KM, Sigmund K: Fairness Versus Reason in the Ultimatum Game. IIASA Interim Report IR- 00-57 (2000). Science 289:1773-1775 (2000).
No. 52 Heino M, Laaka-Lindberg S: Clonal Dynamics and Evolution of Dormancy in the Leafy Hepatic Lophozia Sil- vicola. IIASA Interim Report IR-01-018 (2001). Oikos 94:525-532 (2001).
No. 53 Sigmund K, Hauert C, Nowak MA: Reward and Pun- ishment in Minigames. IIASA Interim Report IR-01-031 (2001). Proceedings of the National Academy of Sciences of the USA 98:10757-10762 (2001).
No. 54 Hauert C, De Monte S, Sigmund K, Hofbauer J: Os- cillations in Optional Public Good Games. IIASA Interim Report IR-01-036 (2001).
No. 55 Ferrière R, Le Galliard J: Invasion Fitness and Adap- tive Dynamics in Spatial Population Models. IIASA Interim Report IR-01-043 (2001). Clobert J, Dhondt A, Danchin E, Nichols J (eds): Dispersal, Oxford University Press, pp. 57-79 (2001).
No. 56 de Mazancourt C, Loreau M, Dieckmann U: Can the Evolution of Plant Defense Lead to Plant-Herbivore Mutual- ism? IIASA Interim Report IR-01-053 (2001). The Ameri- can Naturalist 158:109-123 (2001).
No. 57 Claessen D, Dieckmann U: Ontogenetic Niche Shifts and Evolutionary Branching in Size-Structured Populations.
IIASA Interim Report IR-01-056 (2001). Evolutionary Ecol- ogy Research 4:189-217 (2002).
No. 58 Brandt H: Correlation Analysis of Fitness Land- scapes. IIASA Interim Report IR-01-058 (2001).
No. 59 Dieckmann U: Adaptive Dynamics of Pathogen-Host Interacations. IIASA Interim Report IR-02-007 (2002).
Dieckmann U, Metz JAJ, Sabelis MW, Sigmund K (eds):
Adaptive Dynamics of Infectious Diseases: In Pursuit of Viru- lence Management, Cambridge University Press, Cambridge, UK, pp. 39-59 (2002).
No. 60 Nowak MA, Sigmund K: Super- and Coinfection:
The Two Extremes. IIASA Interim Report IR-02-008 (2002).
Dieckmann U, Metz JAJ, Sabelis MW, Sigmund K (eds):
Adaptive Dynamics of Infectious Diseases: In Pursuit of Viru- lence Management, Cambridge University Press, Cambridge, UK, pp. 124-137 (2002).
No. 61 Sabelis MW, Metz JAJ: Evolution Management: Tak- ing Stock - Relating Theory to Experiment. IIASA Interim Report IR-02-009 (2002). Dieckmann U, Metz JAJ, Sabelis MW, Sigmund K (eds): Adaptive Dynamics of Infectious Dis- eases: In Pursuit of Virulence Management, Cambridge Uni- versity Press, Cambridge, UK, pp. 379-398 (2002).
No. 62 Cheptou P, Dieckmann U: The Evolution of Self- Fertilization in Density-Regulated Populations . IIASA In- terim Report IR-02-024 (2002). Proceedings of the Royal Society of London Series B 269:1177-1186 (2002).
No. 63 Bürger R: Additive Genetic Variation Under Intraspe- cific Competition and Stabilizing Selection: A Two-Locus Study. IIASA Interim Report IR-02-013 (2002). Theoret- ical Population Biology 61:197-213 (2002).
No. 64 Hauert C, De Monte S, Hofbauer J, Sigmund K: Vol- unteering as Red Queen Mechanism for Co-operation in Pub- lic Goods Games. IIASA Interim Report IR-02-041 (2002).
Science 296:1129-1132 (2002).
No. 66 Dercole F, Rinaldi S: Evolution of Cannibalistic Traits: Scenarios Derived from Adaptive Dynamics. IIASA Interim Report IR-02-054 (2002). Theoretical Population Bi- ology 62:365-374 (2002).
No. 67 Bürger R, Gimelfarb A: Fluctuating Environments and the Role of Mutation in Maintaining Quantitative Genetic Variation. IIASA Interim Report IR-02-058 (2002). Geneti- cal Research 80:31-46 (2002).
No. 68 Bürger R: On a Genetic Model of Intraspecific Com- petition and Stabilizing Selection. IIASA Interim Report IR- 02-062 (2002). Amer. Natur. 160:661-682 (2002).
No. 69 Doebeli M, Dieckmann U: Speciation Along Environ- mental Gradients. IIASA Interim Report IR-02-079 (2002).
Nature 421:259-264 (2003).
No. 70 Dercole F, Irisson J, Rinaldi S: Bifurcation Analysis of a Prey-Predator Coevolution Model. IIASA Interim Report IR-02-078 (2002). SIAM Journal on Applied Mathematics 63:1378-1391 (2003).
No. 71 Le Galliard J, Ferrière R, Dieckmann U: The Adaptive Dynamics of Altruism in Spatially Heterogeneous Populations.
IIASA Interim Report IR-03-006 (2003). Evolution 57:1-17 (2003).
No. 72 Taborsky B, Dieckmann U, Heino M: Unex- pected Discontinuities in Life-History Evolution under Size- Dependent Mortality. IIASA Interim Report IR-03-004 (2003). Proceedings of the Royal Society of London Series B 270:713-721 (2003).
No. 73 Gardmark A, Dieckmann U, Lundberg P: Life- History Evolution in Harvested Populations: The Role of Nat- ural Predation. IIASA Interim Report IR-03-008 (2003).
Evolutionary Ecology Research 5:239-257 (2003).
No. 74 Mizera F, Meszéna G: Spatial Niche Packing, Char- acter Displacement and Adaptive Speciation Along an En- vironmental Gradient. IIASA Interim Report IR-03-062 (2003). Evolutionary Ecology Research 5:363-382 (2003).
No. 75 Dercole F: Remarks on Branching-Extinction Evolu- tionary Cycles. IIASA Interim Report IR-03-077 (2003).
Journal of Mathematical Biology 47:569-580 (2003).
No. 76 Hofbauer J, Sigmund K: Evolutionary Game Dynam- ics. IIASA Interim Report IR-03-078 (2003). Bulletin of the American Mathematical Society 40:479-519 (2003).
No. 77 Ernande B, Dieckmann U, Heino M: Adaptive Changes in Harvested Populations: Plasticity and Evolution of Age and Size at Maturation. IIASA Interim Report IR- 03-058 (2003). Proceedings of the Royal Society of London Series B-Biological Sciences 271:415-423 (2004).
No. 78 Hanski I, Heino M: Metapopulation-Level Adaptation of Insect Host Plant Preference and Extinction-Colonization Dynamics in Heterogeneous Landscapes. IIASA Interim Report IR-03-028 (2003). Theoretical Population Biology 63:309-338 (2003).
No. 79 van Doorn G, Dieckmann U, Weissing FJ: Sympatric Speciation by Sexual Selection: A Critical Re-Evaluation.
IIASA Interim Report IR-04-003 (2004). American Natu- ralist 163:709-725 (2004).
No. 81 Ernande B, Dieckmann U: The Evolution of Pheno- typic Plasticity in Spatially Structured Environments: Implica- tions of Intraspecific Competition, Plasticity Costs, and Envi- ronmental Characteristics. IIASA Interim Report IR-04-006 (2004). Journal of Evolutionary Biology 17:613-628 (2004).
No. 82 Cressman R, Hofbauer J: Measure Dynamics on a One-Dimensional Continuous Trait Space: Theoretical Foun- dations for Adaptive Dynamics. IIASA Interim Report IR- 04-016 (2004).
No. 83 Cressman R: Dynamic Stability of the Replicator Equation with Continuous Strategy Space. IIASA Interim Report IR-04-017 (2004).
No. 84 Ravigné V, Olivieri I, Dieckmann U: Implications of Habitat Choice for Protected Polymorphisms. IIASA Interim Report IR-04-005 (2004). Evolutionary Ecology Research 6:125-145 (2004).
No. 85 Nowak MA, Sigmund K: Evolutionary Dynamics of Biological Games. IIASA Interim Report IR-04-013 (2004).
Science 303:793-799 (2004).
No. 86 Vukics A, Asbóth J, Meszéna G: Speciation in Mul- tidimensional Evolutionary Space. IIASA Interim Report IR-04-028 (2004). Physical Review 68:041-903 (2003).
No. 87 de Mazancourt C, Dieckmann U: Trade-off Geome- tries and Frequency-dependent Selection. IIASA Interim Re- port IR-04-039 (2004). American Naturalist 164:765-778 (2004).
No. 88 Cadet CR, Metz JAJ, Klinkhamer PGL: Size and the Not-So-Single Sex: Disentangling the Effects of Size on Sex Allocation. IIASA Interim Report IR-04-084 (2004). Amer- ican Naturalist 164:779-792 (2004).
No. 89 Rueffler C, van Dooren TJM, Metz JAJ: Adaptive Walks on Changing Landscapes: Levins’ Approach Extended.
IIASA Interim Report IR-04-083 (2004). Theoretical Popu- lation Biology 65:165-178 (2004).
No. 90 de Mazancourt C, Loreau M, Dieckmann U: Under- standing Mutualism When There is Adaptation to the Partner.
IIASA Interim Report IR-05-016 (2005). Journal of Ecology 93:305-314 (2005).
No. 91 Dieckmann U, Doebeli M: Pluralism in Evolutionary Theory. IIASA Interim Report IR-05-017 (2005). Journal of Evolutionary Biology 18:1209-1213 (2005).
No. 92 Doebeli M, Dieckmann U, Metz JAJ, Tautz D: What We Have Also Learned: Adaptive Speciation is Theoretically Plausible. IIASA Interim Report IR-05-018 (2005). Evolu- tion 59:691-695 (2005).
No. 93 Egas M, Sabelis MW, Dieckmann U: Evolution of Specialization and Ecological Character Displacement of Herbivores Along a Gradient of Plant Quality. IIASA Interim Report IR-05-019 (2005). Evolution 59:507-520 (2005).
No. 94 Le Galliard J, Ferrière R, Dieckmann U: Adaptive Evolution of Social Traits: Origin, Trajectories, and Corre- lations of Altruism and Mobility. IIASA Interim Report IR- 05-020 (2005). American Naturalist 165:206-224 (2005).
No. 95 Doebeli M, Dieckmann U: Adaptive Dynamics as a Mathematical Tool for Studying the Ecology of Speciation Processes. IIASA Interim Report IR-05-022 (2005). Journal of Evolutionary Biology 18:1194-1200 (2005).
No. 97 Hauert C, Haiden N, Sigmund K: The Dynamics of Public Goods. IIASA Interim Report IR-04-086 (2004). Dis- crete and Continuous Dynamical Systems - Series B 4:575- 587 (2004).
No. 98 Meszéna G, Gyllenberg M, Jacobs FJA, Metz JAJ:
Link Between Population Dynamics and Dynamics of Dar- winian Evolution. IIASA Interim Report IR-05-026 (2005).
Physical Review Letters 95:Article 078105 (2005).
No. 99 Meszéna G: Adaptive Dynamics: The Continuity Ar- gument. IIASA Interim Report IR-05-032 (2005).
No. 100 Brännström NA, Dieckmann U: Evolutionary Dy- namics of Altruism and Cheating Among Social Amoebas.
IIASA Interim Report IR-05-039 (2005). Proceedings of the Royal Society London Series B 272:1609-1616 (2005).
No. 101 Meszéna G, Gyllenberg M, Pasztor L, Metz JAJ:
Competitive Exclusion and Limiting Similarity: A Unified Theory. IIASA Interim Report IR-05-040 (2005).
No. 102 Szabo P, Meszéna G: Limiting Similarity Revisited.
IIASA Interim Report IR-05-050 (2005).
No. 103 Krakauer DC, Sasaki A: The Greater than Two-Fold Cost of Integration for Retroviruses. IIASA Interim Report IR-05-069 (2005).
No. 104 Metz JAJ: Eight Personal Rules for Doing Science.
IIASA Interim Report IR-05-073 (2005). Journal of Evolu- tionary Biology 18:1178-1181 (2005).
No. 105 Beltman JB, Metz JAJ: Speciation: More Likely Through a Genetic or Through a Learned Habitat Preference?
IIASA Interim Report IR-05-072 (2005). Proceedings of the Royal Society of London Series B 272:1455-1463 (2005).
No. 106 Durinx M, Metz JAJ: Multi-type Branching Pro- cesses and Adaptive Dynamics of Structured Populations.
IIASA Interim Report IR-05-074 (2005). Haccou P, Jager P, Vatutin V (eds): Branching Processes: Variation, Growth and Extinction of Populations, Cambridge University Press, Cambridge, UK, pp. 266-278 (2005).
No. 107 Brandt H, Sigmund K: The Good, the Bad and the Discriminator - Errors in Direct and Indirect Reciprocity.
IIASA Interim Report IR-05-070 (2005). Journal of Theoret- ical Biology 239:183-194 (2006).
No. 108 Brandt H, Hauert C, Sigmund K: Punishing and Ab- staining for Public Goods. IIASA Interim Report IR-05-071 (2005). Proceedings of the National Academy of Sciences of the United States of America 103:495-497 (2006).
No. 109 Ohtsuki A, Sasaki A: Epidemiology and Disease- Control Under Gene-for-Gene Plant-Pathogen Interaction.
IIASA Interim Report IR-05-068 (2005).
No. 110 Brandt H, Sigmund K: Indirect Reciprocity, Image- Scoring, and Moral Hazard. IIASA Interim Report IR-05- 078 (2005). Proceedings of the National Academy of Sci- ences of the United States of America 102:2666-2670 (2005).
No. 111 Nowak MA, Sigmund K: Evolution of Indirect Reci- procity. IIASA Interim Report IR-05-079 (2005). Nature 437:1292-1298 (2005).
No. 112 Kamo M, Sasaki A: Evolution Towards Multi-Year
No. 113 Dercole F, Ferrière R, Gragnani A, Rinaldi S: Co- evolution of Slow-fast Populations: Evolutionary Sliding, Evo- lutionoary Pseudo-equilibria, and Complex Red Queen Dy- namics. IIASA Interim Report IR-06-006 (2006). Proceed- ings of the Royal Society B-Biological Sciences 273:983-990 (2006).
No. 114 Dercole F: Border Collision Bifurcations in the Evo- lution of Mutualistic Interactions. IIASA Interim Report IR-05-083 (2005). International Journal of Bifurcation and Chaos 15:2179-2190 (2005).
No. 115 Dieckmann U, Heino M, Parvinen K: The Adaptive Dynamics of Function-Valued Traits. IIASA Interim Report IR-06-036 (2006). Journal of Theoretical Biology 241:370- 389 (2006).
No. 116 Dieckmann U, Metz JAJ: Surprising Evolutionary Predictions from Enhanced Ecological Realism. IIASA In- terim Report IR-06-037 (2006). Theoretical Population Biol- ogy 69:263-281 (2006).
No. 117 Dieckmann U, Brännström NA, HilleRisLambers R, Ito H: The Adaptive Dynamics of Community Structure.
IIASA Interim Report IR-06-038 (2006). Takeuchi Y, Iwasa Y, Sato K (eds): Mathematics for Ecology and Environmental Sciences, Springer, Berlin Heidelberg, pp. 145-177 (2007).
No. 118 Gardmark A, Dieckmann U: Disparate Maturation Adaptations to Size-dependent Mortality. IIASA Interim Re- port IR-06-039 (2006). Proceedings of the Royal Society London Series B 273:2185-2192 (2006).
No. 119 van Doorn G, Dieckmann U: The Long-term Evo- lution of Multi-locus Traits Under Frequency-dependent Dis- ruptive Selection. IIASA Interim Report IR-06-041 (2006).
Evolution 60:2226-2238 (2006).
No. 120 Doebeli M, Blok HJ, Leimar O, Dieckmann U: Mul- timodal Pattern Formation in Phenotype Distributions of Sex- ual Populations. IIASA Interim Report IR-06-046 (2006).
Proceedings of the Royal Society London Series B 274:347- 357 (2007).
No. 121 Dunlop ES, Shuter BJ, Dieckmann U: The Demo- graphic and Evolutionary Consequences of Selective Mortal- ity: Predictions from an Eco-genetic Model of the Smallmouth Bass. IIASA Interim Report IR-06-060 (2006).
No. 122 Metz JAJ: Fitness. IIASA Interim Report IR-06- 061 (2006).
No. 123 Brandt H, Ohtsuki H, Iwasa Y, Sigmund K: A Sur- vey on Indirect Reciprocity. IIASA Interim Report IR-06-065 (2006). Takeuchi Y, Iwasa Y, Sato K (eds): Mathematics for Ecology and Environmental Sciences, Springer, Berlin Hei- delberg, pp. 21-51 (2007).
No. 124 Dercole F, Loiacono D, Rinaldi S: Synchronization in Ecological Networks: A Byproduct of Darwinian Evolu- tion? IIASA Interim Report IR-06-068 (2006).
No. 125 Dercole F, Dieckmann U, Obersteiner M, Rinaldi S:
Adaptive Dynamics and Technological Change. IIASA In- terim Report IR-06-070 (2006).
No. 126 Rueffler C, van Dooren TJM, Metz JAJ: The Evolution of Resource Specialization Through Frequency- Dependent and Frequency-Independent Mechanisms. IIASA Interim Report IR-06-073 (2006). American Naturalist 167:81-93 (2006).
No. 127 Rueffler C, Egas M, Metz JAJ: Evolutionary Predic- tions Should be Based on Individual Traits. IIASA Interim Report IR-06-074 (2006). American Naturalist 168:148-162 (2006).
No. 128 Kamo M, Sasaki A, Boots M: The Role of Trade-Off Shapes in the Evolution of Virulence in Spatial Host-Parasite Interactions: An Approximate Analytical Approach . IIASA Interim Report IR-06-075 (2006).
Issues of the IIASA Studies in Adaptive Dynamics series can be obtained at www.iiasa.ac.at/Research/EEP/Series.html or by writing to eep@iiasa.ac.at.
Contents
Abstract... 2
Introduction ... 4
Modelling ... 8
Results ... 13
Discussion... 19
References ... 26
Tables ... 31
Figure captions ... 33
Appendix ... 36
Figures ... 39
Title: The role of trade-off shapes in the evolution of parasites in spatial host populations: an approximate analytical approach.
Authors: Masashi Kamoa, Akira Sasakib,c and Mike Bootsd
Affiliations:
a) Advanced industrial science and technology. Research center for chemical risk management. 305-8569, Onogawa 16-1, Tsukuba, JAPAN.
masashi-kamo@aist.go.jp
b) Department of Biology, Faculty of Science, Kyushu University Graduate Schools Fukukoka 812-8581, JAPAN
asasascb@mbox.nc.kyushu-u.ac.jp
c) Evolution and Ecology Program, International Institute for Applied Systems Analysis, Schlossplatz 1, A-2361 Laxenburg, Austria
d) Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield. S10 2TN UK
m. boots@sheffield.ac.uk
LRH: M. KAMO ET AL.
RRH: TRADE-OFF SHAPE AND EVOLUTION OF VIRULENCE
Correspondence author: Masashi Kamo Tel: +81-92-861-8771. Fax: +81-92-861-8904.
Abstract
Given the substantial changes in mixing in many populations, there is considerable
interest in the role that spatial structure can play in the evolution of disease. Here we
examine the role of different trade-off shapes in the evolution of parasites in a spatially
structured host population where infection can occur locally or globally. We develop an
approximate adaptive dynamic analytical approach, to examine how the evolutionarily
stable (ES) virulence depends not only on the fraction of global infection/transmission
but also on the shape of the trade-off between transmission and virulence. Our analysis
can successfully predict the ES virulence found previously by simulation of the full
system. The analysis confirms that when there is a linear trade-off between transmission
and virulence spatial structure may lead to an ES virulence that increases as the
proportion of global transmission increases. However, we also show that the ESS
disappears above a threshold level of global infection, leading to maximization. In
addition just below this threshold, there is the possibility of evolutionary bi-stabilities.
When we assume the realistic trade-off between transmission and virulence that results
in an ESS in the classical mixed model, we find that spatial structure can increase or
decrease the ES virulence. A relatively high proportion of local infection reduces
virulence but intermediate levels can select for higher virulence. Our work not only
emphasises the importance of spatial structure to the evolution of parasites, but also
makes it clear that situations between the local and the global need to be considered. We
also emphasise the key role that the shape of trade-offs plays in evolutionary outcomes.
Key Words: Space, evolution, trade-off shape, approximations, adaptive dynamics,
bi-stability.
Introduction
The evolutionary theory of infectious disease is well developed. Classical "mean-field",
homogeneous mixing models, in which there is no co- or super-infection, predict that
selection will tend to maximise the parasite’s epidemiological R0 (May & Anderson,
1983; Bremermann & Thieme, 1989). R0 is a key epidemiological characteristic and
determines the ability of the disease to spread in a population; it is defined as the
average number of secondary infections caused by an average infected host in a
susceptible host population (see Anderson & May,1991). In order to maximise R0,
evolution should maximise the transmission rate and minimise virulence and recovery
(May & Anderson, 1983; Bremermann & Thieme, 1989). However it is doubtful that
the disease behaviour is completely unconstrained, and we therefore generally assumed
that there is a trade-off from the point of view of the parasite between transmission and
virulence. Higher transmission can only be ’bought’ at the expense of higher virulence
as the processes that lead to increased parasite transmission cause damage to the host
(Mackinnon & Read, 1999). If transmission is increasingly costly in terms of virulence,
models predict the evolution of a finite transmission rate and virulence, otherwise
evolution will maximise transmission and virulence; in both cases maximising R0. The
game theoretical approach to the evolution of parasites that assumes trade-offs between
transmission and virulence, is an established route to predicting the long term evolution
of parasites under a number of circumstances (see Lipsitch & Nowak 1995; Frank 1996;
Gandon, 1998ab; Day 2001; Gandon et al. 2001; Day, 2002abc; Gandon et al., 2002;
Boots & Sasaki, 2003; Day 2003; Day & Burns, 2003; Gandon et al., 2003; Gandon,
2004).
General evolutionary theory assumes that the host population is completely
mixed and that therefore any individual is as likely to infect any one individual as any
another. The assumption of homogeneous mixing in host populations ignores the fact
that certain individuals are more likely to contact and therefore infect others. The
inclusion of such spatial/social structure into host-parasite models has shown that this
more realistic assumption about the structure of host populations has dramatic
implications to the evolution of the parasite. A useful approach to examining the role of
the spatial structure of individual hosts is by using lattice models which are also known
as probabilistic cellular automata: PCA (Sato et al., 1994; Rand et al., 1995; Rhodes &
Anderson, 1996; Boots & Sasaki, 1999; Haraguchi & Sasaki, 2000). This approach
models the spatial relationships of individuals within a population. There is now a body
of theoretical work that shows how important spatial structure is to the evolution of
parasites (reviewed in Boots et al., 2006). For example, Haraguchi & Sasaki (2000)
showed that the epidemiological R0 is not maximized when spatial structure is
considered because that parasite transmission to neighbouring hosts is constrained. This
effect on transmission is a result of a form of ‘self shading’ where parasite strains with
lower transmission rates gain an advantage in terms of an increased chance of
susceptible individuals being next to infected ones and therefore available for infection.
Boots & Sasaki (1999) included both local and global transmission and showed that the
ES transmission rate reduced as infection became more local when there is a linear
trade-off between transmission rate and virulence. These models, that range between the
completely local and the global ‘mean-field’ are useful in that they allow us to
understand over what range of spatial variation these effects are important.
However, these studies are largely based on Monte-Carlo simulations of
spatially explicit host-parasite models. The simulations often take a time to reveal
evolutionary trends and sometimes they fail to find the actual evolutionary direction, in
particular, when the selection is weak. Recently, Boots et al. (2006) developed a pair
approximation technique for examining evolutionary stability, which allows the rapid
analysis for the evolution of parasite’s traits. In this paper, we consider a spatial SI
model in which the pathogen transmission can occur both locally and globally and
analyse the evolutionary outcomes by using the approximation technique. Our aims are
firstly to more fully understand how evolutionary stable virulence changes as the
proportion of global infections are increased by assuming the same linear trade-offs as
in Boots & Sasaki (1999). Secondly we wish to examine how different forms of the
trade-off affect the evolution of parasites in spatially structured populations. In
particular, we will examine the role of spatial structure when there is a non-linear
trade-off between transmission and virulence, where the parasite pays an accelerating
cost in terms of virulence from increased transmission. This is likely to be a reasonable
assumption in many microparasitic infections and, although rarely studied, there is
experimental evidence for this decelerating trade-off in nature (see Mackinnon & Read
(2004) for review). In addition, this form of trade-off is commonly assumed in classical
mean field theory since it leads to a finite evolutionarily stable (ES) transmission rate
and virulence and may therefore explain the existence of evolutionarily stable non-zero
virulence in mixed populations. Here we examine how a decelerating trade-off affects
the outcome once there is spatial structure.
Modelling
We, first, mathematically formulate the changes of host densities in time, then analyze
evolutionary outcomes using an adaptive dynamics techniques. These results are
compared with those from the full Monte-Carlo simulation, which has been the
approach of earlier studies (e.g., Boots & Sasaki, 1999). For ease of comparison, we
follow the model by Boots & Sasaki (1999) by considering a regular network of sites,
each of which contains one of a single susceptible individual (S), an infected individual
(I) and empty (O). Susceptible individuals reproduce at a rate r into the nearest
neighbouring sites. They are infected by contact with an infected host at a rate β.
Transmission can occur both locally and globally. When the transmission occurs
globally, a susceptible individual contacts an infected host which is chosen randomly
from one of the sites in the lattice. When the transmission is local, it has a contact to the
nearest neighbouring cell. Global transmission occurs a certain proportion denoted by L
(0≤L≤1). The natural death rate of individuals is d, and infected hosts have an
increased mortality due to infection (virulence: α ). Infected individuals do not
reproduce or recover.
The population dynamics on the lattice is described as, P Ý OO =2[dPSO+(d+αI)PIO−r(1−θ)qS/OOPOO],
P Ý SO =dPSS+(d+αI)PIS−dPSO+r(1−θ)qS/OOPOO
−[r{θ+(1−θ)qS/OS}+βI{LρI +(1−L)(1−θ)qI/SO}]PSO,
P Ý SS =2[−dPSS+r{θ +(1−θ)qS/OS}PSO−βI{LρI +(1−L)(1−θ)qI/SS}PSS],
P Ý IO =dPIS+(d+αI)PII −(d+αI)PIO−r(1−θ)qS/OIPIO
+βI{LρI +(1−L)(1−θ)qI/SO}PSO,
P Ý IS = −dPIS−(d+αI)PIS−βI[LρI +(1−L){θ+(1−θ)qI/SI}]PIS
+r(1−θ)qS/OIPIO +βI{LρI +(1−L)(1−θ)qI/SS}PSS,
P Ý II = −2(d+αI)PII+2βI[LρI +(1−L){θ+(1−θ)qI/SI}]PIS. (1)
where x Ý denotes a time derivative of x. The quantity qσ/σ′σ′ = Pσσ′σ′′ /ρσ represents the
conditional density of σ in the neighbourhood of σ′ site of σ′σ′′ pair. Here we
denote the transmission rate and virulence of wild type pathogen by βI and αI.
Throughout this paper, we use ordinary pair approximation (Matsuda et al. 1992), i.e.,
we approximate the conditional triplet densities by their doublet densities
(qσ/σ'σ'' ≈qσ/σ' for any σ,σ',σ''∈{O,S,I}). This is less accurate approximation than the
other sophisticated ones (e.g., Sato et al. 1994; Keeling 1999), but it is often used to
analyse the lattice model in many other ecological context (Harada and Iwasa 1994;
Kubo et al, 1996; Nakamaru et al. 1997, 1998; Iwasa et al., 1998; van Baalen and Rand
1998; also see some chapters in Dieckman et al. 2000). We will show later in
Discussion that the pair approximation fails to accurately predict the host densities and
the ESS virulence, but is good enough to understand the general tendency of the
evolutionary outcomes.
The global density of infected host (ρI = PI0+PIS +PII) changes with time as
ρ Ý I =
[
βI{LρS+(1−L)qS/I}−(αI +d)]
ρI, (2)where ρS = PS0+PSS +PSI is the global density of susceptible hosts. The definition of
parameters and variables are in Table 1 and 2.
A mutant strain (J) can invade a population at the endemic equilibrium with
resident strain (I), if λ(J|I)= 1
ρJ dρJ
dt =βJ{Lρ ˆ S+(1−L) ˆ q 0S/J}−(αJ +d)>0, (3) where βJ and αJ are the transmission rate and virulence of the mutant. ˆ ρ S denotes
the global density of susceptible host at an equilibrium and ˆ q 0S/J is the local density of
susceptible host in the neighbourhood of the mutant parasite at a “quasi equilibrium”
(Boots & Sasaki 1999; Keeling 1999). In order to obtain the value of ˆ q 0S/J, we assume
that the conditional densities in the nearest neighbourhood of a rare mutant strain
change much faster than the global density of the resident strain, which can be justified
during the initial phase of invasion in which the global density of mutant-infected hosts
remains small. The changes of these fast variables are approximately described as,
q Ý O/J =(d+αJ)qJ/J +(d+αI)qI/J +dqS/J −r(1−θ)qS/OqO/J
+βJ[LρS(qO/S−qO/J)−(1−L){(qO/J −(1−θ)qO/S}qS/J],
q Ý S/J = −dqS/J +r(1−θ)qS/OqO/J −βJ(1−L)θqS/J
−βJ[LρS+(1−L)qS/J]qS/J +βJ[LρS+(1−L)(1−θ)qS/J]qS/S
−βI[LρI +(1−L)(1−θ)qI/S]qS/J,
q Ý I/J = −(d+αI)qI/J−βJ[LρS+(1−L)qS/J]qI/J
+βJ[LρS+(1−L)(1−θ)qS/J]qI/S+βI[LρI +(1−L)(1−θ)qI/S]qS/J,
q Ý J/J = −(d+αJ)qJ/J +2βJ(1−L)θqS/J −βJ[LρS+(1−L)qS/J]qJ/J. (4)
Note that variables without J are at the endemic equilibrium and are constant. We can
solve Eqs. (4) numerically to obtain the quasi equilibrium value of ˆ q 0S/J and then
calculate the invasibility of mutant strain from Eq. (3). When we repeat the procedure
for a various combination of resident and mutant parameters, we can draw pair wise
invasibility plots (PIPs). The PIP is a graphical representation of the evolutionary
outcomes developed in the adaptive dynamics framework (Geritz et al., 1997, 1998). In
the following section, we will analyze the invasibility of mutant strains by drawing PIPs
with trade-offs between transmission rate and virulence.
We also carry out full Monte Carlo simulations where we consider a model
where each site of the lattice is either empty, occupied by a susceptible, or occupied by
an infected. A 100 x 100 regular lattice with a periodic boundary is assumed so that
each site has 4 nearest neighbours. The detail of the simulation has been described
elsewhere (see, for example, Boots et al., 2006). In order to produce PIPs by simulation,
we first carry out a Monte-Carlo simulation in the absence of mutant strains. After the
host densities reach equilibrium, mutation occurs on 10% of the infected hosts, then we
continue the simulation. After a long period, if the mutant strain persists in the
population, we consider that the invasion has been successful. The number of successful
invasions among 20 replicates is represented by a grey scale. For the purposes of this
paper the ESS values predicted by the simulation are assumed to be the correct value.
Since we use approximations to draw PIPs by our analysis, we might expect this to be
less accurate than the simulations.
Results
At first we assume the same linear trade-off relationship assumed in Boots & Sasaki
(1999) such that,
β=3α (5)
and examine how well pair approximations predict the outcome of the Monte-Carlo
simulations. With the linear trade-off, the evolution always leads to higher virulence in
well mixed populations (L=1.0); however, as has been reported previously (Boots &
Sasaki, 1999; Haraguchi & Sasaki, 2000), there is possibility for an evolutionarily stable
(ES) virulence when the population is spatially structured. Figure 1 shows approximate
PIPs with L=0.0, 0.3 and 1.0. The PIPs confirm the results obtained in previous studies,
with global reproduction, the strains with higher virulence always invade (Fig. 1C),
while continuously stable strategies (CSS), defined as the strategies which are both
evolutionarily stable and convergence stable, are predicted once there is local
transmission (Fig. 1A and B).
From Eq. (2) and Eq. (3), if we assume that the virulence is different between
resident (I) and mutant (J) and other parameters are common, the invasion condition can
be written as, 1
R0(αI)− 1 R0(αJ)
⎛
⎝ ⎜ ⎞
⎠ ⎟ +(1−L)( ˆ q 0S/J −q ˆ S/I)>0, (6) R0 is a basic reproductive ratio. If we assume that virulence between strains are close
(say αI =α and αJ =α+ Δα, Δα>0), we can define the selection gradient as,
D(α)= Δ(1/R0) /Δα+(1−L)Δq ˆ 0S/J/Δα. (7)
where
Δ(1/R0)≡ 1
R0(αI)− 1 R0(αJ)
⎛
⎝ ⎜ ⎞
⎠ ⎟ , (8)
Δq ˆ 0S/J ≡( ˆ q 0S/J −qS/I). (9)
If the absolute value of D(α) is large, selection occurs rapidly and if it is positive,
strains with larger virulence can invade. In the limit of L→1, the invasion condition is
the same as that in the well mixed ‘mean-field’ model. As soon as we have spatial
structure (i.e., L<1), the probability of the occurrence of susceptible individuals in the
neighbourhood of an infected individual affects the evolution.
When the trade-off is linear, R0 is a monotonically increasing function of α and
hence Δ(1/R0) /Δα is always positive. Figure 2A shows the dependencies of Δ(1/R0) /Δα and Δq ˆ 0S/J/Δα when L=0. Δq ˆ 0S/J/Δα is negative when a is very small
and gradually goes up as α is increased, but remains in negative over the range
examined. D(α) is also shown in Fig. 2A. It is positive with small virulence and as
virulence is increased, it becomes negative. This indicates that there is an ES virulence.
Since D(α) changes its sign from positive to negative as virulence is increased, the ES
virulence is locally stable.
The way in which D(α) varies with the proportion of global transmission (L) is
shown in Fig. 2B. As is shown in Boots & Sasaki (1999), ESS virulence is increased
with larger L. However, when L is beyond a certain threshold value LC ( 0.3<LC <0.4),
D(α) does not become negative for any α. This indicates a disappearance of ES
virulence and therefore evolution leads to the highest virulence. The linear trade-off
gives small virulence with small L and infinite virulence when L=1.0 as ESS values.
When we see changes of the ES value as a function of L, it first gradually goes up, but
at some point, the ESS virulence jumps up to the infinite values discontinuously.
Between L =0.3 and 0.4, there is an evolutionary bi-stability. Fig. 2C shows the
selection gradient when L =0.35. The selection gradient crosses the horizontal axis
twice. Both of these points can be an ESS, but left one (closed circle) is convergence
stable and the right one (open circle) is convergence unstable; therefore, if evolution
begins at a relatively high value of virulence, the virulence goes toward infinity. If
evolution starts with a lower value, it converges to a finite ESS virulence. Figure 3A
shows a PIP when L =0.33 which shows an evolutionary bi-stability: there is a finite
local CSS virulence, but when the initial virulence of population is greater than a
convergence unstable evolutionary singular point, it evolutionarily diverges towards
infinity. The ES virulence disappears as we increase L, as a saddle node bifurcation
occurs when unstable and stable equilibria collide and disappear (Fig. 3B). If we
compare Fig. 3 with Fig. 1, we can understand how ES virulence varies as a function of
L. The area of bistability depends on the parameters. Figure 4A shows other example of
bistability with different trade-off constant (Eq. 5). The evolutionary trajectories of
Monte-Carlo Simulation is in Fig. 4B.
It is important to note here that the selection gradient above the unstable ESS value is
small. This means that the selection pressure is rather weak and it may therefore be
difficult to observe these evolutionary trajectories in Monte-Carlo evolutionary
simulations. This is due to the fact that the simulations have both mutation and
demographic stochasticity which may swamp the weak selection pressure. Simulation
studies, such as Boots & Sasaki (1999), may therefore conclude that there is a
continuously increasing ES virulence until we have 100% global infection.
Next we consider the importance of a non-linear trade-off between transmission and
virulence such that,
β=Clog(α+1) (10)
where C is a constant. This monotonically increasing, but decelerating trade-off gives a
finite ESS transmission value in completely mixed populations. This form of
decelerating trade-off is commonly assumed in mean-field theory and reflects the
situation where transmission becomes acceleratingly costly to the parasite in terms of
increased virulence. Figure 5 depicts six PIPs with different proportions of global
transmission. The three top panels show PIPs drawn analytically. The other three panels
are PIPs which are drawn by Monte-Carlo simulations.
In Fig. 5, the two panels on the right indicate the result when the proportion of
global transmission is 1 (equivalent to completely mixing). There is an ES virulence
around α=0.145 and the two PIPs are almost identical indicating that the approximate
analysis is nearly exact in the completely mixed infection case. Note that the case L=1
is not equivalent to completely mixing model, as host reproduction to vacant sites
occurs only locally. The two middle panels show the result with intermediate levels of
global infection (L=0.7). Both panels show that the ESS virulence is slightly higher than
the one when L=1.0. The final two panels on the left indicate the result with L=0.0. Both
panels show that there is an ES virulence and the values are almost the same (i.e., the
analytical method predicts the actual ESS well). Boots et al. (2006) have shown that the
prediction of ESS virulence using pair approximation failed with completely local
model if there is no trade-off between virulence and transmission rate. However, if we
assume a trade-off, the pair-approximation predicts the ESS values very well even in the
completely local model. Figure 6A shows detailed analysis of the ESS virulence by the
pair approximation (lines) and the Monte-Carlo simulations (dots). The analytically
predicted ESS is always lower than the results by simulations (Fig. 6A). In Fig. 6B, we
investigated the densities of SS and OO pairs. The pair approximation predicts SS pair
well; however, the density of OO pair is quite underestimated (this has been already
pointed out by Sato et al. 1994). The discrepancy between analysis and simulation may
be attributed to this effect.
Disscussion
We have demonstrated again how spatial structure can have a number of important
consequences to the evolution of parasites. By developing an approximate analytical
technique in addition to Monte-Carlo simulation, we have been able to gain a number of
key insights into the role of spatial structure and trade-off shapes in determining ES
transmission and virulence. In addition, we have demonstrated that spatial structure can
lead to bi-stability in ES transmission and virulence in a parasite system without
acquired immunity. In general, the analysis that we have developed allows a more
detailed understanding of the sometimes complex implications of spatial structure.
It is well understood that the evolution of particular fitness traits may be
constrained by trade-offs with other life history traits (Roff, 2002). In addition, a key
prediction of life-history theory is that the evolution of a particular trait is not just the
result of the trade-off but it is also critically dependent on the functional form of the
trade-off relationship (Roff, 2002). The recent advent of adaptive dynamical
evolutionary theory has further emphasized that it is not only the absolute strength of
costs and benefits that are important, but also how these relationships change under
different conditions (Geritz et al., 1998; Bowers & White, 2002; Bowers et al., 2005).
This adaptive evolutionary theory recognises that trade-off relationships are unlikely to
be exactly linear and that the shape of the relationship is important in determining the
ultimate evolutionary outcome. In particular, the way that the costs and benefits vary
determines both the convergence stability of the evolutionary system and whether
evolutionary branching will occur (Boots & Haraguchi 1999). We have shown that the
shapes of trade-offs are also important in spatial models. With a linear trade-off,
increased local infection always selects for decreased transmission and virulence.
However with a decelerating trade-off, spatial structure can also increase ES
transmission and virulence. Highly local transmission does select for lower virulence,
but intermediate levels of local interaction lead to the higher transmission and virulence
than in a completely mixed population. It is well known that these two trade-off
assumptions have very different outcomes in the mean-field: one leads to maximum
transmission and virulence while the other selects for an intermediate ES. Our work
emphasises that the way in which they interact with spatial structure is also different. It
is important, therefore, in this and most likely in other contexts, that assumptions of
trade-off shapes are examined before the implications of spatial structure can be
completely understood.
The effect of spatial structure on the evolution of parasites can be understood by
examining the selection gradient for virulence evolution, D(α) defined in Eq. 6. It is
divided into two components. The first term corresponds to the maximization of the
basic reproductive ratio, and therefore corresponds to the selection in a conventional
mean-field theory. The second term (dqS/I/dα), involves the local density of infecteds
next to susceptibles and is important due to spatial structure. It is not always possible to
maximize these two terms independently. The first term of Eq. 6 is always positive
when the trade-off is linear and therefore, always selects for higher virulence. The
second term may be negative, particularly at when infection is highly local and therefore
may balance this selection pressure. When the trade-off is non-linear (Eq. 10), there is
an optimum virulence in the absence of spatial structure and therefore the first term is 0
and the direction of the evolution is determined by the sign of the second term.
A number of studies have previously shown that spatial structure can limit the
evolution of transmission without trade-offs (Rand et al., 1995; Haraguchi & Sasaki,
2000; Kamo & Boots, 2004). However, here we have also shown that some degree of
local interaction can select for higher rather than lower transmission and virulence. The
